When I was replacing those heater hoses, the one multipurpose hose, from vaccuum operated valve to core, was hard as a rock. I had to cut it off, and the metal barbed portion of the valve to which it was attached, was rusted badly. the other 5/8" barb on the valve is plastic. When i bent the hose opposite the bend it had hardened into, it took a lot of effort and then when it started to go made a crusty cracking then a snapping sound. It really deteriorated significantly in the last 10 months, as I doubt I would have returned it to service had I noticed how hard it had become on last December's flush, but it is possible.
The inside of the heater valve was about 1/4 occluded with white scale just downstream of the internal flapper. I've no recollection of ever replacing it, it could be original, in use for ~30 years. Vaccuum still moved the flapper but with vaccuum applied, which closes it, to shut off flow when in OFF or MAX AC modes, there is still a lot of flow, when I'd blow through it. I'm told the valve is not designed to completely cut off flow, to keep the AC's evap coil from frosting up, but my factory service manual indicates the flow from fan blows first through Evap coil first, then heater core, so I see no point in this valve not stopping all flow in the OFF or Max positions.
I reassembled without the valve, and blocked off the vaccuum line. My AC is non functioning/non charged anyway. The valves are cheap enough. I can return a new one easily enough, but have just not bothered, yet, and its possible I might not bother ever.
I did flush and backflush the core again. Not much in the way of black flakes came out into the cleaned white bucket. I didn't collect any before data on the heat output at vents, and its been way too hot to use the heater anyway just to test if and to what degree the heater is more effective, post flush.
I used no product, just limited hose water pressure, back and forth twice, about 9 gallons worth and let it siphon itself mostly dry before attaching the new heater hose to intake and water pump and core. The new hose is supple enough I can easily reverse the flow through the core, but unless I find the heat inadequate this winter, I doubt I'll bother.
But that pesky unused Evap core is still in the HVAC box, restricting the blower motor's flow, increasing amperage draw of the motor at higher speeds.
Perhaps I should plumb it with coolant, have a super Alpha roaster heater, as the factory service manual's diagrams make it appear that it has many times the surface area of the heater core, which several years ago was pretty dang impressive on its own. Really though I live in California at sea level, not North Dakota. I will likely cross the Rockies this winter though, nice to have a good heater.